In conventional rocket engines, a main propellant injector sprays liquid propellants into a combustion chamber, where the propellants are burned. The burned propellants expand in an expansion nozzle, where the propellants increase in velocity and produce thrust. A thrust chamber encompasses both the combustion chamber and the expansion nozzle.
One of the propellants (usually the fuel) flow through coolant tubes or channels in the thrust chamber. The relatively cool propellant flowing in the coolant tubes or channels cools the thrust chamber and prevents the thrust chamber from failing or melting. These conventional fluid cooled engines are typically called regeneratively cooled engines because the engine uses one of the main propellant to cool the thrust chambers. Examples of regeneratively cooled engines are the Space Shuttle's SSME engine and the Apollo program's F-1 engine.
The thrust chambers of conventional regeneratively cooled engines include large numbers of individual coolant tubes, perhaps dozens to as high as one thousand coolant tubes, and above. The coolant tubes are brazed or welded together side-by-side like asparagus, or the coolant tubes cooling channels are fabricated from large, thick metal shells that require extensive machining, custom tooling, and custom processes to fabricate the fluid cooling channels (i.e. passages) in the thrust chamber. These types of coolant tubes are produced by a small number (perhaps several) of very specialized, high-overhead, expensive fabricators. The cooling system of the thrust chamber is very often a large part of a rocket engine's procurement expense and requires long lead time to manufacture.
In addition, the material temperature inside of the thrust chamber is often typically near or above 1300 degrees Fahrenheit, requiring the thrust chamber to be made of exotic, rare, or expensive materials.